Docs: Replace links to '/stable/' with '/latest/'. (#5901)
Docs: Replace links to '/stable/' with '/latest/'. (#5901)

* Replace links to '/stable/' with '/latest/'.

* DRY out duplicated 'README.rst' vs. 'docs/index.rst'.

* Include websecurityscanner in docs.

Toward #5894.

googleapis/python-bigtable,googleapis/python-bigtable